The present invention relates to propellants and more particularly to burn rate modifiers for double base propellants.
A double base propellant has an energetic polymer, generally nitrocellulose, plasticized into a gel by an energetic plasticizer, generally nitroglycerine. Various additives are also included in the propellant to improve the mechanical or ballistic properties of the propellant. One such additive is termed a burn rate (or ballistic) modifier which alters the inherently high dependence of the burning rate on chamber temperature and especially chamber pressure.
The objective in burn rate modification of double base propellants is to obtain plateau or mesa burning over a desired range of pressure and burning rate level. These terms come from the shape of a log-log plot of the burn rate equation for double base propellants which is given as r=CP.sup.n or n log P+log C, wherein r is the burn rate, P is the combustion chamber pressure, C is a constant for a given propellant composition at a specific temperature, and n is a constant for on modified propellants but is a variable in modified propellants. Double base propellants with no burn rate modifiers have a constant slope, n, with a value around 0.8 to 0.9. The addition of burn rate modifiers lowers the slope and changes the burn rate over a certain range of pressure. Plateau type propellants are characterized by the pressure exponent n being less than 0.2 in certain regions of pressure. A well defined plateau would have the pressure exponent n being zero over a useful pressure range. Mesa type propellants are characterized by the pressure exponent n being less than zero in certain regions of pressure. These propellants are also relatively temperature insensitive over wider ranges of pressures. As such, it is possible to design a rocket motor or gas generator which provides steady gas output regardless of bulk temperature. Examples of such mesa type propellants are described in U.S. Pat. No. 3,138,499 by Camp, et al.
Unfortunately, the burn rate modifiers used in Camp et al ('499) must include lead salts. Double base propellant processing utilizing lead based compounds poses a hazard to the environment and to personnel in the workplace. The precursor to propellant is a water wet paste which is partially dried and plasticized into a colloidal sheet by rolling between heated calenders. It is likely that some amount of the lead compound is lost in the excess water during the rolling process and subsequently carried into the waste stream. While collection and treatment methods can help clean the wastewater and are in place for any foreign material that may enter the waste stream, the best approach is to replace the problematic compound. The lead hazards also exist for propellant scrap disposal and demilitarization of units. The use of lead salts also leads to health hazards caused by lead oxides in the exhaust gases.
What is needed is a method of producing plateau and mesa propellants without the use of lead compounds.